Two-part base construction for incandescent lamps is known such as disclosed in U.S. Pat. No. 3,852,631, assigned to the assignee of the present invention. In said prior art base constructions, the individual base parts have been formed in two half sections made of insulating material and which are joined together generally with external fastening means along the longitudinal lamp axis. While this type base construction facilitates ease of assembly during lamp manufacture, the length of the base parts formed in half sections often leads to physical distortion when these parts are manufactured as well as poor registration between the half sections during lamp manufacture.
The above indicated problems with two-part lamp base constructions are increasingly severe when the base is constructed with a ceramic material. More particularly, the firing of a ceramic material increases physical distortion so that controlling physical dimensions of a half section base part becomes increasingly difficult and which can lead to failure of the half sections to meet properly when joined together. The advantages of two-part base constructions are thereby largely lost when a ceramic material is required and which has resulted in use of a single base member requiring that a central cavity be machined in said member to physically support the vitreous lamp envelope.
The aforementioned difficulties are reduced with the two-part base construction described in the above referenced Ser. No. 405,650 application. As therein described, the central cavity in this base construction is formed without machining when the two base parts have been assembled together and physical dimensions are better controlled. On the other hand, such improved dimensional control again becomes difficult with increased size of the base member especially in the direction of the lamp longitudinal axis. This problem is particularly severe when the base member is fabricated with ceramic material since the distortion occurring during the firing cycle often produces an open joint at the butt-sealed intersection. Poor dimensional control or distortion of the ceramic lamp base can also seriously impede efforts to "prefocus" the lamp filament during manufacture. Specifically, it is often desirable to accurately position said filament in the final lamp assembly along the longitudinal lamp axis by a predetermined distance. This objective can be achieved far more reliably with precise dimensional control of the ceramic lamp base in a manner hereinafter more fully described. It would be desirable, therefore, to further reduce this overall problem again in a manner not requiring undue modification or additional cost in either lamp or lamp base manufacture.